A writer/director whose film debut, a black comedy called The Guard, opens the Edinburgh film festival next week. Brendan Gleeson stars as an acid‑dropping, evidence-destroying, un-PC cop in a sleepy Galway town.

He's not by any chance related to Martin McDonagh, director of the Brendon Gleeson-starring black comedy In Bruges, is he?

They're brothers. Martin has an exec producer credit on The Guard. "That basically meant he gave Brendan Gleeson the script and got him to read it," John says. Don Cheadle co-stars as a humourless FBI agent parachuted in to smash an international cocaine smuggling ring.

How does The Guard compare to In Bruges?

It's cut from the same cloth, certainly. The script contains some cracking one‑liners. Here's an ex-IRA man to Gleeson, surprised to learn the Provos recruited gay men: "Sure, it was the only way we could successfully infiltrate MI5." The film belongs to the unstoppable Gleeson: deadpan, antagonistic and fractionally melancholic. McDonagh says he could relate to the character's end‑of‑his‑tether state of mind.

Why? Did he used to be an acid-dropping Galway cop?

No. He was born and brought up in London, by Irish parents. But for years he struggled as a screenwriter. "I had got to that point in the film business – I was so pissed off and angry that all of that stuff came out in a sublimated way."

What next?

Directing Gleeson again, in film about a small-town priest whose community turns on him: "It's basically Bresson with gags."